 public class ListNode {
      int val;
      ListNode next;
      ListNode() {}
      ListNode(int val) { this.val = val; }
      ListNode(int val, ListNode next) { this.val = val; this.next = next; }

       @Override
       public String toString() {
             return "ListNode{" +
                     "val=" + val +
                     ", next=" + next +
                     '}';
       }
 }
